Concrete Class in Java

What is a Concrete Class?

• A concrete class in java is a class that has all its methods implemented. For any class to be classified as concrete, it cannot have any unimplemented methods.
• Concrete classes can extend abstract class in Java or implement interfaces, but they must implement all the methods of the abstract class or interface they inherit.
• We can create an object of concrete class using the new keyword.
• A concrete class is an abstract class if it has even one abstract method, i.e. an unimplemented method because it doesn't meet the condition of being a concrete class.
• Concrete classes implement all methods and, in most cases, they inherit an abstract class or implement an interface. Java's final class is a class that cannot be inherited, so it's possible to declare concrete classes as final classes.
• As a concrete class inherits or implements all the methods from an abstract class or an interface, it can also be called a 100% implemented class in Java. Java methods from concrete classes are inherited or completely implemented from abstract classes or interfaces, which helps achieve 100% abstraction.

Syntax of Concrete Class in Java

A concrete class in Java is any such class that has the implementation of all of its inherited members either from an interface or abstract class Here is the basic syntax of the Concrete class:

Here, C is a concrete class that extends abstract class A and implements interface C. Class C has implemented methods from both class A and interface B.

Simple Concrete Class

Now, let's see how we can implement a simple concrete class using java code.

Output:

In the above code, the ConcreteCalculator class has 4 methods that perform arithmetic operations. All four methods add, subtract, multiply, divide are implemented and return the result. As all the methods of the class have been implemented ConcreteCalculator is a concrete class.

Example

The image below shows 4 classes Shape, Circle, Triangle, and Square. The shape is an abstract class with two unimplemented methods area(), perimeter(). The other three classes extend class Shape and implement both the methods area() and perimeter() of the abstract class. Hence, the classes Circle, Triangle, and Square are concrete classes.

Concrete Class Which Extends an Abstract Class

Before seeing the code implementation of a concrete class that extends an Abstract class let's understand what an abstract class means.

Abstraction in Java:

Abstraction is a process of hiding implementation and showing only essential details to the user. Abstraction is used in the case of banking applications to hide crucial details from end-users.

Abstract Class in Java:

• Abstract class in java is a class that is declared using the abstract keyword.
• It contains abstract methods which means it has incomplete or unimplemented methods.
• Abstract classes can have abstract as well as non-abstract methods.
• We cannot create objects of an abstract class to use abstract class methods abstract class needs to be inherited.

Program:

Let's consider a scenario. The shape is an abstract class with two unimplemented methods area(), perimeter(). The other three classes extend class Shape and implement both the methods area() and perimeter() of the abstract class. Hence, the classes Circle, Triangle, and Square are concrete classes.

Let's implement the above example of concrete classes using the java programming language.

Output:

In the above code, Shape is a abstract class with abstract methods area() and perimeter(). Classes Circle, Triangle, and Square inherit the abstract class shape and implement both abstract methods. As the classes Circle, Triangle and Square have all the methods implemented they are Concrete Classes.

Concrete Class Which Implements an Interface

First, let's understand the Interface in java. Interface:

• Interface in Java is a mechanism used to achieve abstraction.
• Interface is said to be a blueprint of a class. It contains only abstract methods and variables.
• Interface doesn't have a method body.
• Interface is used to achieve abstraction. It can also be used to implement multiple inheritances.

Program:

Output:

In the above code DemoInterface is an interface implemented by the abstract class DemoAbstract. The concrete class DemoConcrete implements the methods inherited from both the interface and the abstract class.

Java Abstraction vs. Concrete Classes